It is known to provide double-worm or double-screw systems in which two screws or worms interdigitate or interleave with one another in an elongated housing in which these worms or screws extend through respective bores collectively forming a cavity of a figure-eight cross section.
Apparatus of this type is used for the processing of plastics and synthetic resins, i.e. for the plastifying thereof, to transform pieces of the plastic material into a liquefied or flowable plastified state. Since the plastified product is ejected forceably at an end of the housing, such apparatus can be referred to as an extruder, although the apparatus may be used for blending, mixing, compounding and like processes in addition to liquefying or plastifying a mass.
For the purposes of the present disclosure, all such apparatus will be referred to as extruders and, in particularly, as double-screw extruders, the screws and worms of which are rotated in opposite senses or in the same senses.
The extruder may be designed to shape an elongated workpiece, e.g. by forcing the plastified material through an extrusion die, or simply as a compounder to produce synthetic resins of various kinds and compositions. It may be employed at an input side of a mold to deliver flowable material for injection into the mold of an injection molding apparatus.
Double-worm or double-screw extruders of the aforedescribed types generally have had one piece or multiple piece housings. In a one-piece housing, the housing along the cavity is seamless whereas in a multipiece housing, the housing is assembled, usually by bolting, from a number of parts.
With rotation of the screws or worms in the same sense or in opposite senses, the processed material is forced along the cavity and is subjected to intensive shear action and pressure to effect liquefaction.
The mass of material which is displaced can include solids with or without fillers which can have an abrasive action against the housing walls.
Furthermore, the walls are subject to considerable wear where they are engaged by the flights, threads or turns of the screws, or where the screws draw abrasive solids or suspensions of abrasive fillers in the liquefied mass along these walls. The continuous wear during use of the walls of the cavity has increased in recent years with the tendency to use greater proportions of abrasive fillers and even corrosive additives in the processed synthetic resins.
The wear of the wall of the cavity becomes a life-determining factor for the extruder housing.
It is thus important to minimize such wear and increase the useful life of the extruder housing.
This has been done heretofore by lining the housing with a hardening material, e.g. a so-called hard facing material, or by applying other special coatings to the cavity walls. Hardenable materials can include hardenable steels which can be hardened by nitriding or ion-nitriding processes. The coating of self-flowing hard alloys of an iron, nickel or cobalt base as applied by thermal spraying, for example, or plasma deposition welding, rotation coating and the like has also been proposed. It is also possible to provide wear-resistant inner liners which are preformed and then inserted into the housing body.
In all of the earlier techniques, however, complete satisfaction could not be obtained for a variety of reasons. For the most part, crack formation, pore formation and like defects could arise in the layer. Frequently, moreover, distortion was a problem.